Conventional portable mowing machines having a prime mover, such as an internal combustion engine, have a handle means for supporting and moving the mowing machine so as to cut weeds, grass or the like as desired. The handle means is connected to a pipe which covers a shaft provided for transmitting the rotational torque of the prime mover to a rotary blade or a cutter unit. However, the rotational system extending from the engine to the rotary blade vibrates when the mowing machine operates, and therefore, the vibrations are transmitted via the handle means to the hands and arms of the operator. The vibrations from a mowing machine raise a serious problem since the operator of such machines is apt to suffer from an occupational disease.
According to one of the conventional techniques, which is disclosed in a Japanese Utility Model Publication No. 48-24600, a means for preventing vibrations from being transmitted to a handle means is provided on a pipe which covers a shaft connected between a prime mover and a rotary blade. The vibration proof means comprises a block of rubber mechanically connected via flange means between the outer surface of the pipe and one end of a secondary pipe which covers a portion of the first mentioned primary pipe. The handle means is connected to the secondary pipe so that the vibrations from the primary pipe are prevented from being transmitted to the handle means. However, this arrangement requires a complex construction, and therefore, the total weight of the mowing machine tends to be great.
While the above mentioned conventional technique was made to prevent the transmission of vibrations by blocking or attenuating the vibrations, there is another possibility for preventing vibrations. Namely, if the occurrence of vibrations in a mowing machine were prevented or if the intensity of the vibrations were very low, there would be no problem so that there is no need to block or attenuate the vibrations.
The vibrations in a mowing machine are caused by various factors, such as the direct vibrations from the prime mover, irregular rotations of the rotary blade, inaccuracy in the manufacturing precision of the rotational system, the resonance due to the transverse vibrations of the entire mowing machine, and the resonance due to the torsional vibrations in the rotational system. Among the above listed factors, the resonance due to the torsional vibrations is a potent cause so that the vibrations caused by this resonance have to be reduced in order to reduce the entire vibrations.
According to a conventional technique for reducing this resonance phenomenon the diameter of the shaft is made large so that the natural vibration frequency increases. Since this countermeasure is simple and effectively reduces the vibrations due to the above mentioned resonance, this countermeasure is adopted in all of the customary mowing machines. As a result, there is no conventional mowing machine having a shaft the diameter of which is less than 7 millimeters. Namely, the diameter of the shaft used in conventional mowing machines is 7 millimeters or over. As the diameter of the shaft increases, the weight thereof necessarily increases so that it results in the increase in cost.